.. _env-var-section-ref:

Environment Variables
==============================

The following section lists the environment variables available to configure the cuDecomp library.

CUDECOMP_ENABLE_NCCL_UBR
------------------------
(since v0.4.0, requires NCCL v2.19 or newer)

:code:`CUDECOMP_ENABLE_NCCL_UBR` controls whether cuDecomp registers its communication buffers with the NCCL library using :code:`ncclCommRegister`/:code:`ncclCommDeregister` (i.e., user buffer registration).
Registration can improve NCCL send/receive performance in some scenarios. See the `User Buffer Registration <https://docs.nvidia.com/deeplearning/nccl/user-guide/docs/usage/bufferreg.html>`_
section of the NCCL documentation for more details.

Default setting is off (:code:`0`). Setting this variable to :code:`1` will enable this feature.

CUDECOMP_ENABLE_CUMEM
------------------------
(since v0.5.0, requires CUDA 12.3 driver/toolkit or newer)

:code:`CUDECOMP_ENABLE_CUMEM` controls whether cuDecomp uses :code:`cuMem*` APIs to allocate fabric-registered workspace buffers via :code:`cudecompMalloc`. This option can improve the performance of
some MPI distributions on multi-node NVLink (MNNVL) capable systems.

Default setting is off (:code:`0`). Setting this variable to :code:`1` will enable this feature.